Regenerative medicine and tissue engineering require an accessible, abundant, and reproducible source of cells that fulfill efficacy and safety requirements. During the past decade, basic science and clinical investigators have turned to adipose tissue as a cell source; however, there remains substantial controversy concerning the functional identity of the adipose-derived cells. Are they stromal or stem cells? This is not simply a semantic distinction. A true stem cell has the ability both to self-renew and to differentiate along multiple lineage pathways. While adipose-derived cells have been demonstrated to display these feature in vitro, the critical gold standard experiment, serial transplantation and reconstitution of a functional adipose depot in vivo, has never been reported in the literature. This revised R21 proposal tests our hypothesis that adipose tissue contains adipogenic stem cells in both the heterogeneous stromal vascular fraction (SVF) cells derived by collagenase digest of the adipose tissue and the relatively homogeneous, adherent, culture expanded adipose-derived stromal/stem cells (ASC). Aim 1 will employ the C57BL/6-Tg (UBC-GFP) 30cha/J mouse model as a source of easily trackable GFP+ adipose derived SVF cells or ASCs. Flow cytometry sorted cells will be seeded onto silk scaffolds and implanted into wild type C57BL/6 recipients. After a 4 week period, the constructs will be harvested and the process of transplantation repeated serially. Outcome parameters will be monitored and quantified by flow cytometry, histochemistry, confocal microscopy, scanning electron microscopy, and colony forming unit assays. Serial transplantation of intact adipose tissue explants will serve as a positive control. Aim 2 will perform comparable studies using flow cytometry sorted human SVF cells and ASC transplanted into immunodeficient mice. We anticipate that adipose tissue will prove to contain a stem cell population capable of reconstituting an intact fat pad upon serial transplantation.